Phylum Arthropoda (Arthropods or 'jointed feet')

Arthropods (Phylum Arthropoda, comes from Greek ἄρθρον arthron, "joint", and ποδός podos, "foot") are the largest phylum of animals and include the insects, arachnids, crustaceans, and others. Arthropods are characterized by the possession of a segmented body with appendages on at least one segment. They have a dorsal heart and a ventral nervous system. All arthropods are covered by a hard exoskeleton made of chitin, a polysaccharide, which provides physical protection and resistance to dehydration. Periodically, an arthropod sheds this covering when it grows, in a process called molting.

Arthropods have an open circulatory system. Haemolymph containing haemocyanin, a copper-based oxygen-carrying protein (the copper makes the blood blue, unlike humans that use hemoglobin which uses iron that makes blood red). The blood is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues.

The arthropod body is divided into a series of distinct segments, plus a pre-segmental acron which usually supports compound and simple eyes and a post-segmental telson. These are grouped into distinct, specialised body regions called tagmata. Each segment, at least primitively, supports a pair of appendages.Arthropods also have a complete digestive system with both a mouth and anus.

More than 80% of described living animal species are arthropods, with over a million modern species described and a fossil record reaching back to the late proterozoic era. Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (less than ¼ mm) up to forms several metres (yards) long. The largest living arthropod is the Japanese spider crab, with a leg span up to 3½ metres (12 ft).

Arthropods are typically classified into five subphyla, of which one is extinct:

Trilobites are a group of formerly numerous marine animals that died in the mass extinction at the end of the Permian-Triassic extinction event.
Chelicerates include spiders, mites, scorpions and related organisms. They are characterised by the presence of chelicerae.
Myriapods comprise millipedes and centipedes and their relatives and have many body segments, each bearing one or two pairs of legs. They are sometimes grouped with the hexapods.
Hexapods comprise insects and three small orders of insect-like animals with six thoracic legs. They are sometimes grouped with the myriapods, in a group called Uniramia, though genetic evidence tends to support a closer relationship between hexapods and crustaceans.
Crustaceans are primarily aquatic (a notable exception being woodlice) and are characterised by having biramous appendages. They include lobsters, crabs, barnacles, crayfish, shrimp and many others.

Class: Merostomata (horseshoe crabs - 4 species)
Class: Arachnida (arachnids - 90,000 species)

Order: Acari (mites, ticks - about 47,000 species)
Order: Araneae (spiders - about 35,000 species)

Class: Scorpionida (scorpions - 1,200 species)

Order: Opiliones (harvestmen about 4,000 species)
Order: Pseudoscorpiones (false scorpions - 2,000)

Class: Pycnogonida (Pantopoda - sea spiders 1,100)
Class: Crustacea (crustaceans - about 55,000)

Order: Branchiopoda (brine shrimps, fairy shrimps, water fleas - 1,000)
Order: Copepoda (copepods - about 13,000)
Order: Remipedia (?)
Order: Branchiura (fish lice - 125 species)
Order:Malacostraca (crabs,lobsters,shrimps,
prawns, woodlice,amphipods, mantis shrimps - about 30,000)
Order: Tantulocarida (1)
Order: Cephalocarida (9 species)
Order: Mystacocarida (10 species)
Order: Thecostraca (Cirripedia: barnacles - about 65 + parasitic species)
Order: Ostracoda (mussel shrimps - about 10,000)

Class: Chilopoda (centipedes - 2,500 species)
Class: Diplopoda (millipedes - about 10,000 species)
Class: Symphyla (Symphylida - 160 species)
Class: Pauropoda (500 species)
Class: Entognatha (4,570 species)

Order: Collembola (springtails - about 3,500 species)
Order: Diplura (two-tailed bristle-tails - 800 species)
Order: Protura (proturans - 270 species)

Class: Insecta (insects - about 975,000 species)

Order:Coleoptera (beetles - about 350,000 species)
Order:Strepsiptera (stylops - 530 species)
Order: Hemiptera (true bugs, aphids, scale insects, whiteflies, plant-lice, plant-bugs - about 84,000)
Order: Dictyoptera (cockroaches - about 4,000 species)
Order: Mantodea (mantids - about 1,900 species)
Order: Isoptera (termites, white ants - about 2,300 species)
Order: Neuroptera (lacewings, ant lions - about 4,700 species)
Order: Psocoptera (book-lice, psocids - about 3,000 species)
Order: Zoraptera (30 species)
Order: Grylloblattodea (ice-crawlers - 24 species)
Order: Thysanura (silver-fish, firebrats - 370 species)
Order: Phthiraptera (lice - about 3,500 species)
Order: Archaeognatha (bristle-tails - 350 species)
Order: Odonata (dragonflies -about 6,000 species)
Order: Raphidioptera (snake-flies - 200 species)
Order: Plecoptera (stoneflies - about 2,000 species)
Order: Lepidoptera (butterflies, moths - about 165,000)
Order: Embioptera (web-spinners - 250 species)
Order: Mecoptera (scorpionflies - 500 species)
Order: Orthoptera (crickets, grasshoppers, locusts - about 22,000)
Order: Siphonaptera (fleas - 2,000 specie)
Order: Phasmida (stick-insects, leaf-insects - about 2,500)
Order: Hymenoptera (ants, bees, sawflies, wasps, parasitic wasps - about 150,000)
Order: Dermaptera (earwigs - 1,840 species)
Order: Ephemeroptera (mayflies - 2,100 species)
Order: Megaloptera (alder flies -300 species)
Order: Thysanoptera (thrips -5,100 species)
Order: Trichoptera (caddis flies, sedge flies - about 10,000)
Order: Diptera (true flies, two-winged flies - about 150,000)